Notas

The number of days within which a person must make edits to be considered an "active" user.
In other words, if no edits are made by the user in this amount of time, the user is considered inactive.
Will affect the number shown on Special:Statistics, Special:ActiveUsers and the {{NUMBEROFACTIVEUSERS}} magic word in wiki text.

You might want to leave this as the default value, to provide comparable numbers between different wikis.

Versión antigua

A variable with the same name was present from version 1.13.0 to 1.15.x.
The description was:

This, along with $wgActiveUserEditCount define what User::isActiveEditor() considers to be an active editor.
Simply: if a user makes at least X edits in at least Y days, the user is considered active.
By default, it's set to 30/30, so 1 edit/day.